The 45-year-old New Zealander pleaded guilty to trespass, forcible obstruction of business, destruction of property and illegal possession of a knife. If found guilty Bethune, a member of the Sea Shepherd marine conservation group, could face up to 15 years in prison.

Dressed in a black suit and white shirt and flanked by security guards, Bethune told the Tokyo district court he had had "good reason" to leap aboard the Shonan Maru 2 from a jet ski in February.

"I admit that I boarded the Shonan Maru 2, but I believe I had good reason to do so," Bethune said. "I admit that I fired the butyric acid, but there were additional circumstances that we will discuss in court."

Sea Shepherd claims the whaler was injured when his pepper spray blew back in his face.

Further hearings will take place tomorrow and Monday, and a verdict is expected next month.

The trial opened amid tight security inside and outside the courtroom. About 20 ultra-nationalists called for Bethune to be found guilty and held up banners accusing the "white people" of the international anti-whaling movement of racism. One read: "Don't fuck with the Japanese." Others carried derogatory comments about Bethune.

Shuhei Nishimura, who led the protest, said Bethune deserved a custodial sentence "or else he will act like a wild wolf in the fields and plot another terrorist attack against Japan. To avoid this, we must punish him severely."

Bethune was led into court in handcuffs and with a rope tied around his waist, although both were removed when he took his seat. He smiled when prosecutors showed a photograph of him on a protest boat and described him as a "trademark skinhead".

Japanese whaling officials routinely refer to Sea Shepherd members as "terrorists" and said harassment by the group had forced the fleet to return to port last month with only half its intended catch of 935 whales.

Although the International Whaling Commission (IWC) banned commercial whaling in 1986, Japan is permitted to slaughter whales in the name of "research".

Japan's aggressive pursuit of anti-whaling activists comes amid mounting condemnation of its annual culls in the Antarctic, particularly from Australia and New Zealand.

Two Greenpeace members are also standing trial on theft charges after intercepting a package of whale meat to prove allegations of embezzlement by Japanese whalers. Toru Suzuki and Junichi Sato face up to 10 years in prison if found guilty. A verdict is expected later this year.

Bethune's trial opened as the IWC prepared to meet in Morocco to discuss a controversial proposal that would allow Japan, Iceland and Norway to resume commercial whaling in return for reducing their catch over the next decade.